1. Field of the Invention
This invention pertains generally to the nondestructive examination of metal tubes and more particularly to the nondestructive examination of heat exchanger tubes from the secondary side of a steam generator to characterize wear scars on the heat exchanger tubes that may warrant further examination from the primary side.
2. Description of the Prior Art
Steam generators used in nuclear reactor power plants are very large heat exchangers where heat from a primary fluid heated by a nuclear reactor is transferred to a secondary fluid which is converted into steam and used to drive a turbine generator. Steam generators are housed inside a tall generally cylindrical steel shell. A large numbered of U-shaped heat exchanger tubes are enclosed in the shell and have their ends inserted in holes formed in a horizontal tube sheet or plate near the bottom of the steel shell. The tubes are used to convey the primary fluid which has been heated in the nuclear reactor. The secondary fluid or feed water used to generate the steam is introduced into the steam generator in such a manner that the secondary fluid flows around the outside of the heated tubes thereby converting much of the secondary fluid into steam which is allowed to exit the steam generator through an outlet nozzle at the top of the steel shell.
In the past, steam generator tubing in nuclear plants have been exposed to extreme operating conditions and were susceptible to stress corrosion cracking, mechanical wear, thinning and pitting. To address this susceptibility, a number of techniques have been developed to inspect steam generator tubing for degradation prior to tubing failure in order to prevent forced outages. Steam generator tubing has been most commonly inspected using a variety of eddy current methods, most involving probes which were inserted into the tubes from the underside of the tube sheet on the primary side of the steam generator. The probes were inserted through a steam generated manway in the lower hemispherical inlet and outlet sides of the generator below the tube sheet and into the tube sheet whereby the corresponding tubes were mapped by inserting the probes up through the tubes.
Though highly accurate, the eddy current method of inspecting steam generator tubing is relatively slow and expensive, in that it is time consuming, requires drainage of the primary side of the generator and increases the exposure of inspection personnel to radiation by opening up the primary side of the steam generator.
While there have been a number of attempts to use ultrasonic techniques for inspecting steam generator tubing, as explained in U.S. Pat. No. 5,767,410, in general, most of these techniques used the Lamb ultrasonic wave method of inspection to supplement the eddy current method. A main advantage of the Lamb wave method is that it is not a “spot” technique for tubing inspection as are the eddy current methods. Using Lamb waves, a defect can be detected at relatively long distances from the probe. The range of an ultrasonic Lamb wave probe depends on the wave mode, the information about the defect sought, and the probe design used. The ultrasonic Lamb wave method is attractive because the attenuation of Lamb waves in a metal medium is exceptionally low. The Lamb waves can propagate for a relatively long distance without losing much energy. Lamb waves of a typical amplitude can still be readily detected after traveling a distance of about 10 meters. Another important feature is that Lamb wave propagation is not sensitive to relatively smooth changes in the tubing diameter or the tube bend, such as expansion transition, dents and U-bends.
With the replacement of the majority of the older nuclear steam generators with new designs utilizing thermally treated 1690 many utilities are opting for longer intervals between tubing inspections, thus not having to open the primary side of the steam generator during outages. This has significant cost savings. The maintenance that is performed on the steam generator during intervals between primary side inspections takes place from the secondary side of the unit. Typically, this involves looking for loose parts and/or cleaning of deposits from the secondary side. If a loose part is found, it is removed. However, on occasions where the part is in intimate contact with the tube, wear may have occurred on the tube. Often, the visual inspection techniques available to characterize the wear are incapable of determining the difference between superficial removal of the deposits on the tube and significant wear. The narrow tube lanes, i.e., within the order of 3 mm clearance, makes it extremely difficult to characterize the depth and severity of a wear scar to an extent that would provide sufficient confidence that a wear mark would not develop into a leak. Thus, when a wear scar is identified on the secondary side the primary side of the steam generator must be opened and eddy current or ultrasonic techniques applied from the inside surface of the tube to characterize the depth of the wear scars. This clearly involves significant expense. What is needed is a technique for determining the significance of the wear scars from the secondary side eliminating the need to open the primary side of the steam generator.